In the state of the art relating to secondary switchgear, active protection reacting automatically to an electric fault is not commonly used. This aspect needs special attendance in case of CSS (Compact Secondary Substation) which are placed in public accessible areas. With respect to an arc fault which is accompanied by a sudden and extreme pressure increase, the CSS has to relieve the pressure in order to protect the public and also operators from harm. One of such measures is for example the use of pressure relief openings in the top of the CSS, above human head level. It is well known that in CSS is an arc killer installed inside the tank for the SF6 gas of the ring main unit. This arc killer senses the sudden increase of pressure caused by an internal electric arc inside the tank and reacts by grounding the internal bus bars. The reaction on the pressure increase takes more than 20 ms from arc inhibits so that an explosion is prevented but damage to the ring main unit usually occurs, and extended repair is necessary. Manual reset operation is needed on site to reset the ring main unit. Apart from that, the arc killer is a one-shot mechanism, i.e. when it is tripped it needs service or even replacement. The usual time between the tripping of the arc killer and the re-establishing of the power connection takes at least several hours.
WO 00/67271 concerns an electric switching device for fast closing of a high current in a power network comprising a first electrode, a second electrode, a movable contact element closing said first and second electrodes, and an operating device. The operating device comprises a helically wound first coil secured to the first electrode. The movable contact element comprises a flange making contact with the coil. A current pulse flowing through the coil forms a repulsive force between the coil and the flange, said repulsive force throwing the movable contact element to the second electrode and completing the closing operation.
CSS containing LV and HV side are normally not true arc safe, due to mismatch of applicable standards. Enclosures around an arc fault may open up and cause impact on working staff. At least an exhaust channel is provided to avoid suffocating from hot gases, however in tight spaces as underground, exhaust channels and pressure relief is hard to arrange.
The object of the invention is to fault protect a secondary switchgear, to make the CSS truly arc proof i.e. to protect the equipment from hazards and to limit the damage. A further object of the invention is to bring the secondary switchgear back in operation after fault situations as soon as possible.